1. Field of the Invention
The present invention relates to novel catalysts, to a process for the preparation of such novel catalysts, and to a process for the elimination of nitrogen oxides contained in a gaseous flowstream by catalytic reduction employing said novel catalysts.
2. Description of the Prior Art
French Patent No. 79/05,598 describes a catalyst for reducing the nitrogen oxides NO and NO.sub.2 (NO.sub.x) contained in a gaseous flowstream to molecular nitrogen, in the presence of ammonia.
This catalyst, consisting of a porous alumina support and a catalytically active phase deposited thereon, has been found to be especially effective for the reduction of residual nitrogen oxides formed during the synthesis of nitric acid by the oxidation of ammonia with oxygen. The catalyst has the added advantage of being suitable for use at relatively low temperatures, on the order of 190.degree. to 250.degree. C., resulting in a saving of energy, as it is not necessary to reheat the gaseous flowstreams to be treated. However, it was discovered that such a catalyst presents certain disadvantages when used for the catalytic/ammonia reduction of the nitrogen oxides contained in gaseous flowstreams which also contain small amounts of the oxides of sulfur.
In effect, it was found that the presence of sulfur oxides, SO.sub.2 and SO.sub.3 (SO.sub.x), in the gaseous flowstream gives rise to a rapid deactivation of the catalyst.
This deactivation may be attributed to the formation of ammonium bisulfate, NH.sub.4 HSO.sub.4, and ammonium sulfate, (NH.sub.4).sub.2 SO.sub.4, by reaction between the sulfur oxides, the water present in the gaseous flowstream and the ammonia that is absolutely necessary for the catalytic reduction of nitrogen oxides.
The ammonium bisulfate is actually deposited in the pores of the catalyst, thereby reducing both catalyst volume and active specific surface area thereof. Furthermore, it too was found that the ammonium bisulfate reacted with the alumina to form the compounds NH.sub.4 Al(SO.sub.4).sub.2 and Al.sub.2 (SO.sub.4).sub.3, which also deactivate the catalyst.
Gaseous flowstreams of this type, containing the nitrogen oxides NO.sub.x and minor amounts of the sulfur oxides SO.sub.x typically comprise vehicular exhaust gases, the residual gases from boilers and other stationary combustion units, such as the gases from thermal power stations, which are already substantially desulfurized.
Catalysts capable of catalytically reducing NO.sub.x under conditions similar to those described above have already been proposed to this art.
These are catalysts, the active phase of which is deposited onto a titanium dioxide support.
However, such catalysts have an optimum activity at temperatures on the order of 300.degree. to 400.degree. C. Furthermore, they are relatively expensive, as titanium dioxide is more costly than alumina. Another disadvantage of the TiO.sub.2 supports resides in their low mechanical properties.